Signing the Transaction but Not the Decision: Whisper Attacks and a Binding Defense for AP2

Research shows AP2 agent-payment signatures can be manipulated into valid but wrong carts; proposed A-VIP defense binds signed intent to purchases.

A study demonstrates Whisper attacks on the AP2 agent payment protocol, where ordinary product-description text steers shopping agents into carts that pass every cryptographic check but no longer match user intent. Using Gemini Flash-Lite models specified by AP2's default sample agents, three attacks succeeded at 90%, 56%, and 73.3%, with the vulnerability spanning seventeen Google models, three agent frameworks, cross-vendor anchors, and Google's consumer assistant. The proposed A-VIP defense treats signed intent as a capability grant, binding credential lookups to sessions and cart lines to seen listings, blocking the first two attacks with zero false positives while surfacing unauthorized spending. The authors release A-VIP code, machine-checked invariants, and AP2-WhisperBench with 1,544 evaluation scenarios.